My argument may not be obvious, but it’s not hard to follow, either. It centers around the notion that security must be evaluated not based on how it works, but on how it fails.

It doesn’t really matter how well an ID card works when used by the hundreds of millions of honest people that would carry it. What matters is how the system might fail when used by someone intent on subverting that system: how it fails naturally, how it can be made to fail, and how failures might be exploited.

The first problem is the card itself. No matter how unforgeable we make it, it will be forged. And even worse, people will get legitimate cards in fraudulent names. …

Not that there would ever be such thing as a single ID card. Currently about 20 percent of all identity documents are lost per year. An entirely separate security system would have to be developed for people who lost their card, a system that itself is capable of abuse. …

But the main problem with any ID system is that it requires the existence of a database. In this case it would have to be an immense database of private and sensitive information on every American—one widely and instantaneously accessible from airline check-in stations, police cars, schools, and so on.

The security risks are enormous. Such a database would be a kludge of existing databases; databases that are incompatible, full of erroneous data, and unreliable. …

What good would it have been to know the names of Timothy McVeigh, the Unabomber, or the DC snipers before they were arrested? Palestinian suicide bombers generally have no history of terrorism. The goal is here is to know someone’s intentions, and their identity has very little to do with that.